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Influence of shock pressure testing on the service life of HDPE piping systems [PE]


Plastic Pipes Conference Association # 2010 Vancouver

Ulrich Schulte, Heinz Vogt, Hans-Friedrich Enderle, Dieter Lilge

The completion of a pressure test is mandatory for all new pipe systems. Its purpose is to demonstrate the overall leak-proof condition of the system. In the course of this test, HDPE pipes of SDR 17 class are exposed to high stress loads. In an extensive test programme, all failure mechanisms observed in long-term hydrostatic (creep rupture) strength testing were examined. The results show that SDR 17 pipes made of PE 100 grade HDPE material will clearly exceed the required 50-year minimum service life even after shock pressure testing.

Before a new piping system is put into service, it must be shock pressure tested according to EN 805 [1], irrespective of the pipe material employed. The aim of this test is to confirm the tightness of the entire system, and specifically of its joints and fittings, at an elevated internal pressure.
Piping systems for the distribution of drinking water must attain a service life of at least 50, if not 100, years. They are typically rated for a 10 bar operating pressure and are predominantly made of HDPE pipes. In the case of performance class PE 100 material, wall thickness class SDR 17 piping will be used to meet this pressure requirement [2]. Since pressure surges in such local supply networks are virtually impossible to compute, the test pressure for the shock pressure test according to EN 805 includes a fixed-rate surge allowance which brings up the test pressure to 15 bar [1]. In accordance with EN 805, this test pressure must act on the tested HDPE piping system for at least 1.5 hrs at a test temperature of 20 °C [1, Appendix A.27].

The above shock pressure test places a high stress on HDPE piping. For a 15 bar test pressure, the boiler formula yields a circumferential stress level of 12 MPa. In the creep rupture strength diagram for a 1.5 h load cycle, this value lies just under the 20°C reference curve for PE 100. This “flat” arm marking the minimum strength requirement is defined by two reference points, viz., 100 h/12,4 MPa (soon to be amended to 100 h/12,0 MPa) and 50 years/10.0 MPa.
Although it is already evident from the foregoing that the shock pressure test will not create unacceptably high circumferential stress in the pipe wall, it is the aim of this paper to review, by means of systematic model trials and practice-based investigations, the potential effects of a shock pressure test on the service lifetime of a HDPE pipe system. To this end, all failure modes observed on HDPE piping – ductile failure, brittle fracture after slow crack growth, brittle fracture in the wake of thermooxidative degradation – will be taken into account.

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